The present invention relates to hetero-junction bipolar transistors which are miniaturized and are able to perform high-speed operation and to semiconductor device using the same.
There are descriptions in Technical Report ED91-161 of IEICE and in IEEE Trans. Electron Devices, vol. ED-34, pp. 2246-2253 (1987), About conventional hetero-junction bipolar transistors.
In the case of a hetero-junction bipolar transistor described in the first document, the junction area of an emitter electrode and an emitter is approximately equal to that of the emitter and a base, and the junction area of the base and a collector is always larger than those of the above-mentioned junctions as shown in FIG. 26. Because of this, there has been a problem that an emitter resistance caused by the contact resistance of the emitter electrode is large and the capacitance between the collector and the base is large. In order to connect a wire to the smallest emitter electrode, the dimensions of a transistor are limited by those of the emitter and it has been difficult to realize a transistor having enough small dimensions.
Furthermore, in the case of a bipolar transistor described in the second document, an emitter electrode is formed on a polycrystalline semiconductor layer as shown in FIG. 27. There has been a problem that in the polycrystalline semiconductor the mobility of electrons is low and it is impossible to make the emitter resistance small sufficiently.